1. Field of the Invention
This invention relates to a color registration correction in an image forming process for a color printing apparatus having a plurality of photosensitive drums. The images are formed toner images of each color and the toner images of each color are transferred directly to a recording sheet or to an intermediate transfer medium then transferred again to the recording sheet from the intermediate transfer medium. This invention avoids the color registration correction caused by fluctuation between rotation speeds of the plurality of photosensitive drums.
2. Description of the Background Art
The color printing apparatus having above mentioned structures, in which a plurality of photosensitive drums are arranged in line, is called a tandem type apparatus. In the tandem type color printing apparatus, each photosensitive drum has a drum gear and a motor rotates the drum gear. The rotation of the drum gear causes the rotation of the photosensitive drum and a toner image is formed on the photosensitive drum.
In the image forming process of the tandem type color printing apparatus, the fluctuation between the rotation speeds of the photosensitive drums causes a color registration difference on the recording sheet or the intermediate transfer medium. The causes of the fluctuation between the rotation speeds are an eccentricity of the drum gear and the photosensitive drum, and an eccentricity of a rotation axis of the drum gear and the photosensitive drum. Accordingly, the fluctuation between the rotation speeds changes periodically, as shown in FIG. 1.
To minimize the color registration difference, in the color printing apparatus disclosed in Japanese laid-open patent 2005-140870, line patterns of each color that have predetermined pitch are formed around the photosensitive drums of each color, and the toner images based on the line patterns are transferred to the recording sheet or the intermediate medium. A sensor detects the transferred toner images, and the position difference between the transferred toner images of each color are calculated based on an output signal of the sensor. Based on the calculated position difference, a relationship of a rotation start position of each photosensitive drum is decided. Maintaining the relationship of rotation start positions minimizes the color registration difference.
Shown in upper portion of FIG. 1, the difference of the speed of each photosensitive drum causes the color registration difference. To control a rotation start position of each photosensitive drum reduces the difference of the color registration shown in lower portion of FIG. 1.
Usually, the above mentioned color printing apparatus has two print modes. One is a black and white mode, which uses only the photosensitive drum for black toner to print a black and white image, and the other is a color print mode, which uses all photosensitive drums to print a color image. In the color print mode, the rotation start positions of all photosensitive drums are controlled to satisfy the relationship shown in the lower portion of FIG. 1. In the black and white mode, only the photosensitive drum for black image is rotated to form a black toner image. After finishing forming the black toner image, the start position of the black photosensitive drum is not controlled. Therefore the start position of the black photosensitive drum does not satisfy the relationship shown in the lower portion of FIG. 1, and the rotation start position needs to be adjusted before starting the next printing.